Certain substances, when bound to the membrane of neurons, cardiac and skeletal muscle, salivary acini, and other excitable cells, behave as molecular probes of membrane potential. The optical properties of these molecules, most notably fluorescence and absorbance, vary in a linear fashion with potential, and may, therefore, be used to monitor action potentials, synaptic potentials, or other changes in membrane voltage from a large number of sites at once, without the necessity of using electrodes, either intracellular or external. We propose to develop more sensitive probes, to extend the technology associated with their use, and to use these "molecular voltmeters" for optical recording of membrane potential from hiterto inaccessible regions of single neurons such as axonal and dendritic processes as well as from many sites simultaneously in small assemblies of neurons and in electrical syncitia, in order to study spatial and temporal patterning of activity, including conduction pathways. First, we intend to develop an apparatus which will be used to exploit the optical properties of potential sensitive dyes and will include an ion laser microscope system to record membrane potential changes from fine processes of single identified neurons; first isolated in tissue culture, and then, in situ within an invertebrate neuropil using intracellular application of the probes. These structures are not penetrable by microelectrodes, and are frequently too far away, electrically, for their activity to be reflected in the somata. Secondly, we propose to extend this system so that it is able to record optically from more than one hundred loci at once, and to use this capability to record the patterns of electrical activity throughout syncitial tissues. Finally, we hope to be able to grow "simple nervous systems -- small assemblies of synaptically connected invertebrate central neurons in tissue culture, and to study the properties of such systems by optically recording electrical activity from all of their components simultaneously.